In the 2016 Science Advances paper, Photo-induced Force Microscopy (PiFM) images of PS-b-PMMA block copolymer (BCP) with a pitch of ~40 nm have been published, clearly illustrating chemically identified structures with spatial resolution of 10 nm. To analyze the sensitivity and spatial resolution of AFM IR instruments, BCP is a great sample because (1) the lamellar structure of the constituents enables verification that the chemically distinct molecules are being measured by acquiring images at the wave numbers associated with the various chemical species, (2) the periodicity of the structure can be controlled precisely by molecular weights; and (3) the lamellar structure is random enough that the acquired image cannot be aided by constructive inference as is the case in some atomic lattice imaging observed on 2D materials in ambient conditions.
PiFM vs. AFM IR Techniques
The spatial resolution of AFM IR instrument is identified by many criteria: (1) effective volumes of the sample and tip that are interacting, (2) detection technique’s sensitivity and (3) background signal that identifies the signal-to-noise. The following table shows a comparison of PiFM with other techniques in these areas:
||Smaller than tip radius; independent of film thickness
||Excellent on all sample thickness (dependent on quality factor Q of cantilever)
||No competing background signal
||Larger than tip radius; grows with film thickness
||Good on thicker samples (> ~ 100 nm)
||Thermal expansion from neighboring material
||Smaller than tip radius; independent of film thickness
||Depends on quality of optics
||Strong far-field scattered signal
Compared to other AFM IR techniques, PiFM has favorable operating conditions on all the criteria given above, leading to its superior performance, both in surface sensitivity and spatial resolution.
PiFM Images for Identification of PS and PMMA Molecules
Since the Science Advances paper, spatial resolution of PiFM has increased to the point where the different chemical blocks of a PS-b-PMMA sample having a pitch of about 22 nm can be resolved. In figure 1, the PMMA and PS molecules are shown in green and red colors, respectively. Cross-sections of the PiFM images for PMMA and PS do not correlate with each other as expected and they indicate the measured pitch to be about 21 nm ( two pitches are measured in the cross-section). Every polymer molecular block with about 11 nm width is imaged clearly and the signal rise measures less than 6 nm, which is the commonly used spatial resolution of the instrument. There are direct ways to raise the quality factor Q of the cantilever and the spatial resolution of PiFM will continue to improve.
Figure 1. PiFM images at 1493 cm-1 (to identify PS molecules, colored in red) and at 1733 cm-1 (to identify PMMA molecules, colored in green) along with AFM topography (bottom left) and phase image (bottom right) with cross-section profiles along the line shown in the images; the lines are drawn at the same physical location of the sample and the measured lengths are also from the same physical locations of the sample. One can see that the PS and PMMA line profiles anti-correlate as they should for BCP. The combined chemical image (top right) also confirm the lamellar nature of the BCP. The measured full pitch is 21 nm (the cross-section measures two full pitch).
PiFM Images and PS and PMMA Molecules
Figure 2 is yet another example of PiFM’s capability to map chemical species with extremely high spatial resolution and yield information that AFM alone cannot provide. Here a group of concentric cylindrical shells of PS and PMMA are constructed through directed self-assembly as categorically shown in the bottom right. AFM topography illustrates the formation of circular depressions while the phase image shows that the depression and the substrate may be of different materials. However, topography or phase image does not reveal whether the proper cylindrical features have been fabricated. By taking PiFM images to highlight PS (imaged at 1492 cm-1) and PMMA (imaged at 1733 cm-1) molecules, it can be seen that the correct structure has been constructed.
Figure 3 indicates the identification of defective structures using PiFM images. The region detected by dashed rectangle indicates that while the topography looks alike, the structures are made completely by PS and not by any PMMA. Further, it can be seen that the outer PMMA cylinders are not found in all the structures.
Among all the AFM IR techniques, PiFM yields the highest spatial resolution with development prospects because the quality factor of the cantilever can be increased in direct ways in the future. PiFM promises to be a future-proof technique as no special optical detector is needed since the excitation wavelength is changed.
Figure 2. Directed self-assembly (DSA) structure of PS-PMMA is imaged by PiFM. Topography (top left) and phase image (top middle) cannot shed any confirmation of the chemical make-up of the structure. The two PiFM images highlighting PMMA (in green) and PS (in red) molecules confirm the fabrication of the correct chemical make-up. The combined chemical map image (top right) matches the schematic of the structure shown in the bottom right (a slight mismatch in the two PiFM images are due to thermal drift).
Figure 3. Figure 3 PiFM images of DSA structure from a different region reveal that defects can be identified with the aid of PiFM even though AFM topography shows similar features as structures without defect.
This information has been sourced, reviewed and adapted from materials provided by Molecular Vista.
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